This invention relates to reflectance spectrometers, and more particularly to a method and apparatus for instant display and analysis of reflectance spectra for field mineral identification.
Reflectance spectra has been successfully used for identification of many minerals ranging from alunite to zeolite. For example, a portable reflectance spectrometer is disclosed by Dr. Alexander F. H. Goetz, the present inventor, and others, in U.S. Pat. No. 4,043,668. Briefly, the spectrometer disclosed there included an optical unit and a digital recording unit for recording the intensity of reflected radiation at different wavelengths selected by a filter wheel spectral range (0.4 to 2.5 micrometers). The recorded spectral data are later analyzed to determine the composition of the materials which produce the reflectance spectra.
This development of a portable reflectance spectrometer followed the development in 1967-1970 of a Multispectral Photography Experiment S-158 included in the APOLLO 12 mission. That experiment utilized multispectral imaging systems with analysis capability for determination of lunar lithographic boundaries remotely from orbit, but without real-time spectral data analysis. For a description of the experiment see Alexander F. H. Goetz, et al., "Apollo 12 Multispectral Photography Experiment," Geochimica Acta, Vol. 3, 2301-2310, MIT Press, 1971.
Following that development, new research programs were established in 1970-1973 to improve the accuracy of telescopic spectroradiometric imaging systems. The role of computer image processing in orbital multispectral photography was established as a means of enhancement. The first preliminary geologic investigations were undertaken in the field on the Colorado plateau to evaluate and interpret earth satellite (ERTS) multispectral data, suitably enhanced. Studies were also carried out to determine the quality and use of ERTS radiometric information with reference to arid desert regions. See Alexander F. H. Goetz, et al., "Symposium . . . " Mar. 5-9, 1973 NASA SP-327 at pages 403 to 411, and 1159 to 1167. Also Proceedings of the 4th Annual Conference on Remote Sensing in Arid Lands, 136-147, Univ. of Arizona, Tucson, November. 1973.
After an earth applications effort was formally organized at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology, a novel portable reflectance spectrometer was developed for the 0.4 to 2.5 micrometer range, also based on digital recording of reflectance radiation spectra in the field. This instrument is the subject of the aforesaid U.S. Pat. No. 4,043,668, assigned to California Institute of Technology. The electronic recording unit was a separate "backpack" system, with an inherent time delay prior to actual mineral identification. The unit did not incorporate features of the present invention, and had no instant display capability for analysis, but was capable of recording for later analysis about 200 spectra per day on compact digital tape cassettes. Data thus obtained was further processed at a large trailer or other installation using a programmed digital computer. The disadvantage of that system was that mineral identification could not be made on site, thus creating a problem in correlating the results of data analysis with specific locations of the terrain.
Increased activity from 1975-1978 in the field of multispectral imaging and analysis at JPL led to the development of systems with CCD imaging devices, readily interfaced with more rapid computer analysis and readout systems, as is more fully discussed in U.S. Pat. No. 4,134,683, by Alexander F. H. Goetz, et al. An airborne imaging system including several arrays of charge coupled devices (CCD), or linear detector arrays, were used to obtain simultaneously spectral reflectance data at different wavelengths for a target area using a plurality of filters each accommodating a particular bandwidth. Data from the arrays were read into a computer or microprocessor which made it possible to analyze image data in real time, and to display the information superimposed an an image of terrain data to provide an overlay of mineral identification data on geographic data. However, generally speaking, fairly broad visible and near-IR bands were covered, and only rough qualitative analysis of minerals or oil spill zones was possible. The system was not portable and could be programmed to identify the presence of only one specific material at a time.
The instrument of U.S. Pat. No. 4,134,683 included "band ratioing" using divider circuits. "Band ratioing" is a technique which seeks to provide positive identification of materials by measurement or calculation of ratios of the two most prominent spectral peaks, rather than a single peak, characterizing the material. Band ratioing thus creates ratios of two filtered channels to cancel out topographic effects, etc. Band ratioing is also helpful in dealing with the problem of high data correlation between channels caused by systematic effects such as topography.
Later development described in U.S. Pat. No. 4,345,840, involved in ratioing radiometer able to identify selected materials that reflect radiation within a predetermined band. That instrument is particularly suited for differentiating between the clay minerals most commonly found in the earth's terrain. The instrument is a self-contained dual-beam ratioing radiometer with two optical trains directed at the same target. It provides a continuous digital readout of ratio values from the two optical trains each of which includes a separate filter for selection of the narrow spectral bands to be ratioed for identification of the presence of a particular mineral on the basis of known spectral characteristics of the mineral.
In an exemplary embodiment, the narrow bands ratioed are selected infrared and visible bands in the 0.4 to 2.5 micrometer range, and means are provided for pivoting the axis of at least one optical train with respect to the other, in order that both have their axis directed at the target. Each optical train channel has two relay (repeater) lenses with a selectable filter between the lenses, and a detector at the rear.
As a particular feature of the instrument, two coaxial filter wheels serve the separate channels by providing slits in one filter wheel between filters to pass light to the selected filter in the other wheel, and slits in the other filter wheel between filters to pass light already directed through a selected filter on to a relay lens and detector. In that way, one filter wheel can be rotated independently of the other for particular materials to position a selected filter in the light path between relay lenses while a slit in the other filter wheel passes the filtered light through to the second relay filter and detector. Alternatively, both filter wheels may be turned together, as when the paired filters for particular minerals have been selected and properly disposed on the filter wheels. Operation to check for the presence of the different minerals in the target area can then be simplified by stepping both filter wheels together through all positions, for example five, such that for each position each filter wheel presents a different filter paired with a filter in the other wheel.
Continuous ratioing of the two detector output (division of the detector output of one channel by the detector output of the other channel), and continuous digital readout of the ratio for display and/or recording, permits continuous and instantaneous identification of the material in the field using data tables for specific minerals. For example, kaolinite and montmorillonite yield very different ratio values for filters centered at 2.10 and 2.17 micrometers, and hence the presence of either material can be immediately determined in the field. However, in many clay mineral formations, a large number of individual components are present, including silicates, carbonates, and mixed oxides. In order to perform analysis with that band ratioing device, it becomes necessary to have dozens of filters available in the field, and extensive band ratioing data on selected infrared and visible bands for the known minerals. Hence for field prospecting and identification of minerals an analysis system incorporating some or all of the above-described analysis features, but with greater memory capacity has been desired. In addition, a system for more rapid identification of minerals in formations with complex component mixtures has been desired.